Microwave, millimeter-wave, and optical over-the-air telecommunications systems suffer performance degradation due to signal attenuation by scattering and absorption of transmitted electromagnetic energy by air-borne water, such as rain, snow, and fog, as well as performance degradation arising from common-frequency (wavelength) interference, often referred to as co-channel interference. Methodologies and algorithms exist for increasing the transmitter power to overcome path attenuation due to rain and are operational in various satellite and terrestrial radio systems. Thus, many microwave and millimeter-wave terrestrial radio transport and distribution systems, available for 18-, 23-, 24-, 28- and 38-GHz broadband access radio links incorporate link management options affording some form of Automatic Transmitter Power Control (ATPC), and concomitant ATPC initiation and operation methodologies.
One key factor limiting the density of deployment of radio-based systems in a given coverage area is co-channel interference between radio links from nearby radio deployments of the same system. Depending upon line-of-site coupling exposures, co-channel interference may occur from more-distant radio links or radio distribution systems using the same channel, including xe2x80x9cforeignxe2x80x9d systems of other license holders. Because heavy rainfall severely attenuates 38-GHz signals (a result of the near half-wavelength resonant sizes of large raindrops), the current engineering practice for deploying 38-GHz broadband customer-access radio systems that lack power control is to set the transmitter powers high. Often times, the power is increased by a factor up to 10,000 times the minimum power required for acceptable clear-weather communications, irrespective of whether or not there are interfering signals. The resulting potential clear-weather interference generated is so great that co-channel radio links must be separated by large xe2x80x9cfrequency coordinationxe2x80x9d distances, lowering the achievable density of such radio deployments significantly.
There currently exist automatic transmitter power control techniques that can help minimize the interference environment, enabling somewhat closer-separated radio links sharing the same frequencies (channel pairs). For example, preset link-design values of transmitter powers at both ends of a link (e.g., modest, but sufficient powers to assure acceptable clear-weather performance and incidental interference) are increased significantly only during times when it is inferred from some xe2x80x9crelevantxe2x80x9d real-time on-site measurement that a link is experiencing heavy rainfall. The two-way information payload communications are sustained by significantly increased transmitter powers, overcoming the (assumed) rain attenuation, and this same rain attenuation is assumed to eliminate or minimize any interference to neighboring radio deployments.
However, depending upon the automatic transmitter power control methodology employed, certain conditions can arise wherein a link""s transmitter power will increase dramatically even though clear weather surrounds the transmitters, causing potentially devastating interference to nearby co-channel links and to neighboring radio systems: An example is the present-day automatic transmitter power control ATPC algorithm that initiates increased xe2x80x9cfar-endxe2x80x9d transmitter power whenever the perceived signal received at the xe2x80x9cnear-endxe2x80x9d falls below a pre-set value. Should the latter perceived signal weaken significantly or disappear due only to electronic degradation of the xe2x80x9cnear-endxe2x80x9d receiver, and not rain attenuation, then strong interference could result throughout the coverage area from the xe2x80x9cfar-endxe2x80x9d transmitter controlled (inappropriately) to operate at high, and possibly the very possibly highest power. Additionally, the present day automatic transmitter power control techniques can operate to automatically increase transmitter powers at both ends of a radio link when a weakened signal is perceived, resulting in potentially disastrous interference, now doubled in impact, in the absence of actual rain attenuation.
An alternate automatic transmitter power control methodology involves making an on-site determination that a pre-set maximum degradation of the quality of the information payload has occurred, either instead of, or in addition to measuring the received signal strength. Again, depending upon the particular automatic transmitter power control methodology employed, degraded information payload quality may occur for other reasons beside rain attenuation. Such payload information degradation would then trigger high transmitter power at one or both ends of a radio link, causing severe clear-weather interference.
Co-channel radio interference itself is a common cause of degraded information payload quality in radio systems. Such interference could trigger a power increase in other nearby transmitters, again in clear weather. This leads potentially to an-area-wide xe2x80x9crun-upxe2x80x9d of transmitter power and hence, nearly ubiquitous interference, and not only sufficient to degrade or interrupt communications within the intended serving area, but could also interfere with nearby xe2x80x9cforeignxe2x80x9d (i.e., other license holders"") systems. It is the collective interference power from multiple transmitters whose powers are each increased up to 10,000 times that required in the absence of rain that is at issue here.
A similar automatic transmitter power control requirement exists for the so-called xe2x80x9cFree-Space Optical (including infra-red) Communications (FSOC) systems,xe2x80x9d wherein optical transmitter power is be minimized consistent with sustained communications during clear weather to facilitate eye-safety, promote longer laser lifetimes, reduce powering and heat dissipation/cooling requirements, etc. However significant power increases must occur in the presence of thick fog, dense smoke or very heavy rainfall to maintain communications. Again, it is inappropriate to increase optical transmitter power in the absence of high ambient optical attenuation. However, as discussed previously, such power-run up can nonetheless result from utilization of present-day automatic transmitter power control schemes.
Briefly, in accordance with a preferred embodiment of the invention, a method is provided for controlling transmission power of a near-end and far-end transceiver pair in communication with each other. The method commences by determining whether the strength of the signals received at near-end and far-end transceivers are simultaneously attenuated a prescribed value below received signal strength values measured during clear weather free-space conditions. If the received signal strengths measured at the near-end and far-end transceivers are simultaneously attenuated below the prescribed value, then transmission power of the near-end and far-end transceivers is increased by predetermined increments (or sequences or increments) to restore, but not exceed the strengths of the received signals to their respective signal strength values measured during clear weather free-space conditions. Increasing the transmission power of the near-end and far-end transceivers only when the received signal strengths, as measured at the far-end and near-end receivers, respectively, are simultaneously attenuated serves to reduce the likelihood of co-channel interference during clear weather conditions.